ARTICLE IN PRESS

Quaternary Science Reviews 27 (2008) 295–311

Cosmogenic -10 and -21 dating of late Pleistocene glaciations in Nyalam, monsoonal Himalayas

Joerg M. Schaefera,Ã, Peter Oberholzerb,1, Zhizhong Zhaoc, Susan Ivy-Ochsd, Rainer Wielerb, Heinrich Baurb, Peter W. Kubike, Christian Schlu¨chterf

aLamont-Doherty Observatory, Geochemistry, Palisades, NY 10964, USA bETH Zu¨rich, Isotopengeologie, CH-8092 Zu¨rich, Switzerland cChinese Academy of Geological Sciences, Institute of Geomechanics, Beijing, PR China dETH Zu¨rich, Teilchenphysik, CH-8093 Zu¨rich, Switzerland ePaul Scherrer Institute, c/o Institute of Particle Physics, ETH-Hoenggerberg, 8093 Zu¨rich, Switzerland fGeologisches Institut der Universita¨t Bern, CH-3012 Bern, Switzerland Received 10 October 2007; accepted 11 October 2007

Abstract

We present cosmogenic 10Be and 21Ne chronologies from 21 erratic boulders on three moraine sequences in Nyalam county, monsoonal Himalayas, southern margin of the Tibetan Plateau. The surface exposure ages provide evidence for at least two distinct glacial advances during the late stage of the last glacial cycle and for one or more significantly older glaciations. The distribution of cosmogenic ages from the three ridges of the old moraine sequence is inconsistent with their stratigraphic order. Because exposure periods of the erratics prior to deposition on the moraine surface is shown to be small, the chronology–stratigraphy mismatch suggests that the cosmogenic ages do not date moraine deposition but most likely significant moraine ridge denudation and related boulder exhumation after initial deposition of the moraines during the penultimate glacial cycle or earlier. The surface exposure ages based on various currently accepted production rate scaling protocols yield age differences of up to 35% reflecting the poor knowledge of terrestrial cosmogenic production rates at low latitude/high altitude sites. Even within this conservative uncertainty range, our results do not yield evidence for late Pleistocene glaciations in monsoonal Tibet to be asynchronous to those in mid-latitudes on both hemispheres. There is an urgent need to improve the knowledge of terrestrial cosmogenic production rates and their scaling to low latitude regions to fully exploit the climate information archived in tropical moraine sequences. r 2007 Elsevier Ltd. All rights reserved.

1. Introduction Stage-2 (MIS-2: 12–30 ky; Hays et al., 1976). This termination is consistent in timing with the onset of Timing and amplitude of paleoglaciations represent temperature and atmospheric CO2 increase recorded in important cornerstones of terrestrial paleoclimate research, Antarctic ice-cores (Jouzel et al., 2001). However, paleocli- because glaciers are arguably the most sensitive recorders matic patterns in general and the timing of past glaciations of climate changes. Recent studies indicate that the major in particular remain controversial for tropical/monsoonal glaciation at the end of the last glacial cycle referred to as areas. Various studies report earlier Last Glacial Maximum the Last Glacial Maximum terminated near-synchronously glaciations from tropical south America (Smith et al., 2005) in mid-latitudes on both hemispheres between about 17 and Tibet (Phillips et al., 2000; Owen et al., 2001, 2002b; and 19 ky ago (Schaefer et al., 2006), i.e. during Marine Finkel et al., 2003) during the relatively ameliorated Marine Isotope Stage-3 (MIS-3: 30–60 ky). Atmospheric moisture content and monsoon strength were high during ÃCorresponding author. Tel.: +1 845 365 8703; fax: +1 845 365 8155. E-mail address: [email protected] (J.M. Schaefer). warm periods and low during cold periods (e.g. Wang 1Baugeologie und Geo-Bau-Labor, Bolettastrasse 1, 7000 Chur, et al., 1999). According to those latter studies, glaciations Switzerland. were triggered in the tropical Andes and Tibet during warm

0277-3791/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2007.10.014 ARTICLE IN PRESS 296 J.M. Schaefer et al. / Quaternary Science Reviews 27 (2008) 295–311 periods by higher precipitation rates and increased very appropriate to investigate the response of glaciers to moisture transport to high altitude accumulation areas. monsoonal precipitation and larger scale temperature In contrast, recent glaciological studies (Oerlemans, 2001, signals, respectively. We mapped the stratigraphy of the 2005; Anderson and Mackintosh, 2006) argue that glaciers moraine sequence (Figs. 1–3) and dated paleoglaciations are most sensitive to summer temperature changes and that with cosmogenic beryllium-10 (10Be) and neon-21 (21Ne) to precipitation is of rather limited importance as driver of assign chronologies to the paleoglaciatons. paleoglaciations on time-scales of several hundred years or Despite of the dramatic progress in TCN techniques and more. This important conflict illustrates our limited the large number of cosmogenic dating studies (e.g. Owen understanding of the tropics as an element of the Earth’s et al., 2002a, b, 2003, 2005, 2006; Finkel et al., 2003; Zech climate system, which is still under intensive debate (Kane et al., 2005; Abramowski et al., 2006), production rates of and Evans, 2000). A promising approach to resolve this TCNs are poorly know on the Tibetan Plateau due to the debate is to intensify the investigations of paleoglaciations lack of direct production rate calibration experiments and in tropical and monsoonal areas. poorly constraint scaling protocols to high altitude/low We report new terrestrial cosmogenic nuclide (TCN) latitude sites. This is reflected in the significant discrepancy chronologies of late Pleistocene glaciations near Nyalam on among published scaling procedures. To illustrate this the southern margin of the Tibetan Plateau in the monsoonal significant source of uncertainty, we present our exposure Himalayas. Local climate in this area is strongly influenced ages using five distinct and currently accepted scaling by the Indian monsoon system due to an orographic low in protocols. In the following, we only draw conclusions the Himalayan mountain chain. In that sense, this area is consistent with either set of exposure ages.

Fig. 1. Overview of the field region; 1: Puluo Glacier; 2: Puluo moraines; 3: Fu Qu Valley; 4: Fu Qu Glacier; 5: Shisha Pangma Glacier; 6: Shisha Pangma summit; 7: Naisa moraines; the inset in the upper left shows a map of the region in larger scale. For more details about the Nyalam moraines and the Naisa valley and moraines, see Figs. 2 and 3. The picture is courtesy of Prof. William Bowen. ARTICLE IN PRESS J.M. Schaefer et al. / Quaternary Science Reviews 27 (2008) 295–311 297

2. Regional setting (8012 m a.s.l., Fig. 1) since the Pliocene (from youngest to oldest): Little Ice Age, Neoglacial, Puluo (or Qomolangma 2.1. Climate setting and glacial II), Fu Qu (or Qomolangma I), and Nyanyaxiongla (or Nyalam). A similar stratigraphy can be assigned to Naisa Sampling sites are sketched in Figs. 1–3. Nyalam County valley (Fig. 3). Glacial deposits and morphologies are is situated at the southern margin of the Plateau. The excellently preserved in the Fu Qu Valley up river from the climate is dominated by the Indian monsoon, with very village of Nyalam and the Naisa valley. We refer to high summer precipitation rates of up to 8 mm day1, and the older moraine set in the Fu Qu system as ‘‘Nyalam’’, to hardly any winter precipitation (e.g. NASA GPCP V2 the younger as ‘‘Puluo’’ and to the Naisa valley moraines satellite-gauge precipitation data). We investigated glacia- as ‘‘Naisa’’. tions in two valley systems, the Fu Qu (Fig. 2) and the Naisa (Fig. 3) valleys. We sampled the most stable elements 2.2. Geomorphology of the morainic surface sediments, i.e. large erratic boulders on lateral and terminal moraines. 2.2.1. Nyalam moraine sequence The field area is characterized by high mountains, deeply The Nyalam moraines south of Nyalam (Figs. 2a and b) incised valleys and a rather erosive landscape setting. consist of four ridges at an altitude of about 3900 m. These Zheng (1988) lists five major glaciations in Fu Qu valley on lateral moraines from the right side of Fu Qu glacier merge the southern side of the massif around Mt. Xixabangma up valley. Fu Qu glacier currently terminates 20 km further

Fig. 2(a). The Fu Qu valley. Google Earth map of Fu Qu valley: 1: Nyalam moraines (see Fig. 2b for more detail); 2: Nyalam city; 3: Puluo moraines (see Fig. 2c for more detail); 4: Puluo glacier; 5: Fu Qu glacier; 6: Shisha Pangma; the different color-codings of the Google Earth map represent different resolutions, the grayish areas being highest resolution. ARTICLE IN PRESS 298 J.M. Schaefer et al. / Quaternary Science Reviews 27 (2008) 295–311

Fig. 2(b). The Fu Qu valley. Google Earth map of the Nyalam moraine sequence: 1: Ny-1, Ny-2, and Ny-3 are sampled from this outer moraine; 2: Ny-4, Ny-5, Ny-6, and Ny-7 are from this intermediate moraine ridge; 3: Ny-8, Ny-15, Ny-16, and Ny-17 are from this innermost ridge; 4: Nyalam city.

up the valley at an altitude of 5100 m and is receding. The glacier is not covered by extensive supraglacial sediment glacier shows only little supraglacial debris cover. The and is currently receding. It terminates about 7.5 km up moraine ridges rise 300 m above the present-day valley valley at an altitude of about 4700 m. bottom. Crests are rounded and slopes are gentle indicating significant weathering and denudation of the landscape. In contrast, the moraines from the Puluo and Naisa stadium 2.2.3. Naisa moraine sequence display a significantly fresher morphology. We sampled the The Naisa glaciation in Naisa Valley, some 20 km up three inner ridges of this moraine sequence (Fig. 2b). valley of Nyalam (Fig. 1), is represented by a moraine system with a multi-ridged terminal section (Fig. 3a and b) 2.2.2. Puluo moraine sequence at 4200 m altitude. At least three sub-advances are The Puluo glaciations deposited two distinct moraine documented by a compact moraine belt less than 200 m sets at an altitude of approximately 4100 m (Figs. 2a and c). in width indicating different sub-stages of one major glacial The Puluo 2 moraine was deposited inside Puluo valley, event. Lateral and terminal moraines are preserved. We whereas the Puluo glacier most likely merged with the sampled the three largest boulders on the innermost left main system of Fu Qu glacier during the Puluo 1 event. lateral/terminal moraine (Fig. 3b) and the two largest Consequently, only the lateral moraines can be mapped for boulders on the outermost right lateral moraine with the Puluo 1. We sampled erratic boulders on the left lateral goal of dating the youngest sub-stages represented by this moraine sets. The left lateral moraine of the Puluo 1 event moraine belt. Naisa glacier carries little sediment, termi- is about 50 m higher than the Puluo 2 moraine. Puluo nates today at an altitude of 5360 m and is receding. ARTICLE IN PRESS J.M. Schaefer et al. / Quaternary Science Reviews 27 (2008) 295–311 299

Fig. 2(c). The Fu Qu valley. Google Earth map of the Puluo moraine sequence: 1: outer moraine ridge of Ny-9 and Ny-10; 2: inner moraine ridge of Ny-11, Ny-12, and Ny-13; 3: presumably Little Ice Age terminal moraine; 4: Puluo glacier.

The altitudes of the accumulation areas of all glaciers are low enough to avoid the high-altitude desert effects reported by Harper and Humphrey (2003).

2.3. Samples

Very large boulders are abundant on all moraines, well embedded in the (Figs. 2d and 3c). Smaller blocks (diameter less than 1 m) are almost exclusively found as fragments of larger ones. The sizes of the sampled erratics range from 1.5 to 4.5 m height, and up to 7 m length. They are of gneissic lithology, which is less resistant to erosion than, e.g. middle to fine-grained granite. Some of the large blocks have broken apart and considerable parts have fallen off. Whereas the boulders from the Puluo and Naisa moraines appear relatively fresh, many boulders on the Fig. 2(d). The Fu Qu valley. Example of a sampled Nyalam boulder. The Nyalam moraines show weathering pits up to 30 cm erratic Ny-6, the largest block sampled. diameter and 20 cm depth on the top surface. Samples for ARTICLE IN PRESS 300 J.M. Schaefer et al. / Quaternary Science Reviews 27 (2008) 295–311 exposure dating were taken from the top of the most stable erratic boulders from the Naisa moraines (Fig. 3b), three looking part of a boulder, between weathering pits if rocks on the innermost left lateral moraine, about 40 m present and avoiding broken-off parts. To minimize the apart (NAI-1, NAI-2, and NAI-3) and two blocks on the probability of overturning of the erratics, wherever possible outermost right-lateral moraine, about 10 m apart (NAI-4 boulders on the very crests were sampled that were deeply and NAI-5). The samples were between 2 and 7 cm thick. embedded in the soil. We sampled 21 boulders total; eleven from the Nyalam 3. Materials and methods moraine belt, three (Ny-1, Ny-2, and Ny-3) on the outermost ridge, four (Ny-4, Ny-5, Ny-6, and Ny-7) on 3.1. Snowline depression the middle ridge, and four (Ny-8, Ny-15, Ny-16, and Ny- 17) on the innermost ridge (Fig. 2b). Two erratics were To estimate the snowline-lowering underlying the sampled on the Puluo 1 moraine (Fig. 2c) about 30 m apart respective glacial advances, we applied the THAR (Toe (Ny-9 and Ny-10), and three erratics on Puluo 2 about to Headwall ratio) method, assuming a THAR of 0.4 75 m apart (Ny-12, Ny-13, and Ny-14). We sampled five (Meierding, 1982). We are aware that this approach is not

Fig. 3(a). The Naisa valley. Google Earth map of the Naisa moraine sequence: 1, left lateral/terminal Naisa moraines; 2, right lateral/terminal Naisa moraines; 3, historical moraines damming a proglacial lake, 4: Naisa glacier. ARTICLE IN PRESS J.M. Schaefer et al. / Quaternary Science Reviews 27 (2008) 295–311 301

Fig. 3(b). The Naisa valley. Lateral/terminal moraines sampled in Naisa Valley Sampled ridges of the Naisa moraines. Note that the ridge of NAI-1, NAI- 2, and NAI-3 is stratigraphically younger than the moraine ridge we sampled NAI-4 and NAI-5 from.

very precise (Owen and Benn, 2005); however, we consider the THAR method to be the most robust ELA reconstruc- tion method for these glaciers carrying little sediment, given the available field data and maps in this remote region. We base our calculations on Google Earth maps (Figs. 2 and 3) and a 1:100,000 Russian flight map as well as a 1:200,000 map of the Chinese Geological Survey. We concentrate on the Puluo 2 and the Naisa glaciation since the ‘‘Toe’’, i.e. the terminal moraines, is preserved for these two moraine belts only.

3.2. Geochemistry and analytics

Moraine ages were determined by measuring cosmogenic 10Be in all 21 samples and 21Ne in all 11 Nyalam samples and in 4 Puluo samples. Principles and details of Fig. 3(c). The Naisa valley. Example photograph of a sampled boulder. surface exposure dating are described, e.g. in Gosse and Example of a Naisa Valley sample (NAI-1). Picture is taken on the innermost left lateral moraine, approximately 300 m in distance from the Phillips (2001) and Niedermann (2002). terminal moraine. Note the sharp and fresh appearance of the moraine Quartz separates were prepared following a modified ridges. protocol based on Kohl and Nishiizumi (1992) and were ARTICLE IN PRESS 302 J.M. Schaefer et al. / Quaternary Science Reviews 27 (2008) 295–311

10 21 10 analyzed for Be and Ne. The geochemistry for Be was 6th order fit (>20 hPa) done at the cosmogenic dating laboratory at the Lamont- Y= 1.45E-24 x6 – 1.55 E-19 x5 + 7.04E-15 x4 Doherty Earth Observatory, and the measurements were -2.20E-10x3 + 6.06E-6 x2– 1.17E-1 x + 1010 2 carried out on the PSI/ETH tandem AMS facility at ETH 1200 R = 1.000 Zu¨rich (Synal et al., 1997). Neon isotope concentrations were measured with a 901 1000 Series2 sector field static mass spectrometer at the 800 Poly. (Series2) noble gas laboratory at ETH Zu¨rich (Beyerle et al., 2000). This spectrometer features a modified Baur/Signer 600 ion source equipped with a compressor that enhances 400 the sensitivity for and neon by two orders of Airpressure (hPa) 200 magnitude (Baur, 1999). We applied step-wise heating to enrich the cosmogenic neon fraction in the low-tempera- 0 ture steps (400 and 600 1C). Details are given in Table 1b 0 10000 20000 30000 40000 altitude (m) and Fig. 5. Temperature vs elevation (slope of equation is lapse rate, K/m). Note that lapse rate is taken at pressures greater than 150 hPa (below 3.2.1. Production rates and ages tropopause) We present the 10Be ages based on five different, 400 currently accepted, scaling procedures (CRONUS-Earth 300 10Be–26Al webbased age-calculator Version 2.0; http:// 200 Series1 y = -0.0062x +299.78 100 Linear (Series1) hess.ess.washington.edu/math/al_be_v2/al_be_multiple.php; R2 =0.9995 0 Balco et al., in press; see also the consistent excel-add-in for 0 5000 10000 15000 cosmogenic production rate calculations presented by Vermeesch, 2007), four of them taking into account Fig. 4. Airpressure–altitude relation for the field region. (a) ECMWF variations of the geomagnetic field in time (for more ERA-40 re-analysis (25 years average; Kallberg et al., 2004) use a details see Table 2). The 21Ne ages are based on the sea variational data assimilation system to make a new synthesis of the in situ 1 1 and remotely sensed measurements of meteorological parameters such as level/high-latitude value of 20.3 g yr (Nieder- temperature, airpressure, humidity, wind field etc., made over the period mann, 2000). To calculate the production rate at the since 1957. The produced spatial resolution has a grid-spacing close to respective sample altitude, we used the scaling factors for 125 km in the horizontal. The NCEP/NCAR (http://www.cdc.noaa.gov/ only, because there is no evidence that muonic cdc/reanalysis/reanalysis.shtml) re-analysis yields indentical results for our production is relevant for cosmogenic noble gases. We case (agreement of both re-analysis should be good outside polar regions). Airpressure as a function of elevation (m) for the field area. Actual applied a correction for the deviation of the regional air airpressure is about 12 hPa higher at an altitude of 4000–4500 m than pressure–altitude relation from standard atmosphere based predicted by standard atmosphere, resulting in a 8% lower production on the ECMWF Re-Analysis ERA40 data (25 years rate (see online Table 1). Dataset used is ERA40 from 1977 to 2001 (25 average; Kallberg et al., 2004; Fig. 4). This correction years average). The data are output at 23 pressure levels in the yields about 12 hPa higher local air pressure than predicted atmosphere, from 1 to 1000 hPa; the sixth order polynomial (420 hPa) shows the best fit from about 6000 m down to sea level (o2 hPa error); by standard atmosphere and consequently 8% lower other fits are not as good; (b) temperature (K) as a function of elevation production rates for our field region. This type of (m). Slope yields atmospheric lapse rate. correction is significant and should be applied everywhere in tropical and sub-tropical regions. Corrections of the production rate for shielding by surrounding landscape were not required for the Nyalam samples and were smaller underneath the boulders, and compare these data to those than 3% for the Puluo and Naisa samples (Table 1). of the fully exposed surface samples of the same boulder Corrections for boulder symmetry as suggested by Masarik (see also Lifton et al., 2001 for a similar approach). The and Wieler (2003) are not applied because a more recent approximate nuclide concentration due to in situ cosmic study comparing TCN concentrations in samples of ray production in the partially shielded samples can be different geometric positions and shape does not find this calculated. The difference of this calculated value to the effect (Balco and Schaefer, 2006). measured concentration of TCN basically allows to All our conclusions drawn below are independent of the quantify pre-exposure of the boulder and possibly to choice of scaling procedures. recognize other inherited contributions such as 21Ne (Section 5). This was done for 6 of the 11 boulders 3.3. Evaluation of potential periods of pre-exposure from the older Nyalam moraines. These bottom samples were analyzed for cosmogenic neon. It was not feasible to To identify pre-exposure and non-cosmogenic contribu- collect shielded samples large enough for beryllium tions in a rock surface (for short referred to here as ‘‘non-in measurements. situ’’), we collected control samples being partially shielded To find out if the cosmogenic 21Ne excess in the shielded from cosmic rays in their present position, e.g. from samples originates entirely from in situ exposure on the ARTICLE IN PRESS J.M. Schaefer et al. / Quaternary Science Reviews 27 (2008) 295–311 303 moraines or partly from pre-exposure or nucleogenic Wieler (2003), assuming that the decrease of production production, we estimate the expected amount of in rate with the column length is caused by the decreasing situ produced cosmogenic neon in the bottom sample contribution of backscattered particles. Because the deter- relative to the surface sample. If the expected concentration mination of shielding depths is rather uncertain, the agrees within errors with the measured concentration, overall error of the flux calculations is estimated to be on substantial non-in situ produced neon can be excluded. the order of 30%, except for Ny-15 where the error is We used simplified boulder geometry to calculate the assumed to be 50%. secondary flux J at the position of the shielded sample relative to the top. The mean shielding depth d due to overlying rock was estimated for 4. Results 51 sectors above the horizontal plane (around one horizontal axis only, i.e. assuming infinite extension of Results are given in Tables 1a–c (sample parameters and the block along this axis). For each sector, the neutron flux measured 10Be and 21Ne concentrations) and Table 2 (10Be J was calculated as: and 21Ne ages of surface samples), as well as Fig. 5 (neon 10 21 ld m 3-isotope plots), Fig. 6 ( Be and Ne ages comparison), Jðd; yÞ¼J0 e sin y, (1) and Fig. 7 (probability plots of the 10Be ages). Table 2 where d is the depth from rock surface, y is the angle presents the 10Be-ages based on five different scaling 10 26 measured from the horizon, and J0 is the flux in vertical protocols (CRONUS-Earth webbased Be– Al age cal- direction. The parameter l is the attenuation length culator, Version 2.0, http://hess.ess.washington.edu/math/ for collimated unidirectional cosmic radiation, which al_be_v2/al_be_multiple.php; Balco et al., in press), show- is about 1.3 times the attenuation length for cosmic ing that the difference in exposure ages due to distinct ray production rates (Dunne et al., 1999). We use production rate scaling is significant between the scaling l ¼ 200 g cm2. The exponent m has been taken as 2.3 procedure assuming a constant magnetic field (column (Nishiizumi et al., 1989). The fluxes from all sectors were ‘‘Lal/Stone constant’’ in Table 2) and the protocols then summed up and normalized to the flux at an including a correction for the geomagnetic field changes unshielded position. Finally, the total flux at the shielded in time (all others columns in Table 2). These differences location was corrected for particles backscattered from are up to 35% for the ages older than 40 ky, about 10% for the surrounding ground. These corrections are less the ages around 20 ky (10%) and small for the youngest than 5%. They were deduced from Fig. 1 in Masarik and ages of about 10 ky (5%). On the other hand, the

Table 1a Cosmogenic 10Be and 21Ne concentrations, coordinates, and other parameters for all samples

Sample Latitude Longitude Elevation Airpressure Thickness Density Shielding Erosion rate [10Be] 1s [21Ne] 1s (DD) (DD) (m) (hPa) (cm) (g cm2) correction (cm yr1) (atoms g1) (atoms g1) (atoms g1) (atoms g1)

NY-1 28.133 85.967 3902 634 3 2.7 1 0.0001 2.743E+06 1.097E+05 1.058E+07 1.110E+06 NY-2 28.133 85.967 3902 634 3 2.7 1 0.0001 2.514E+06 1.282E+05 1.190E+07 1.039E+06 NY-3B 28.133 85.967 3902 634 4.5 2.7 1 0.0001 5.364E+06 1.663E+05 2.496E+07 2.420E+06 NY-4 28.15 85.967 3840 639 2.5 2.7 1 0.0001 3.292E+06 1.679E+05 2.496E+07 2.420E+06 NY-5 28.15 85.967 3885 635 3 2.7 1 0.0001 7.972E+06 2.790E+05 3.215E+07 2.448E+06 NY-6 28.15 85.967 3941 631 3.5 2.7 1 0.0001 1.497E+07 5.538E+05 4.996E+07 7.344E+06 NY-7 28.15 85.967 3976 628 5.5 2.7 1 0.0001 2.806E+06 2.049E+05 1.709E+07 1.880E+06 NY-8 28.15 85.967 3905 634 3.2 2.7 1 0.0001 7.711E+06 3.316E+05 3.118E+07 1.383E+06 NY-9 28.185 85.935 4301 604 1 2.7 0.978 0.0001 1.005E+06 6.128E+04 5.05E+06 3.030E+06 NY-10 28.183 85.933 4301 604 2 2.7 0.978 0.0001 1.576E+06 7.723E+04 n.a. n.a. NY-12-1 28.168 85.933 4261 607 2.5 2.7 0.978 0.0001 6.654E+05 2.928E+04 3.89E+06 3.624E+06 NY-12-2 28.168 85.933 4261 607 2.5 2.7 0.978 0.0001 6.558E+05 2.623E+04 n.a. n.a. NY-13 28.168 85.933 4273 606 4 2.7 0.978 0.0001 7.095E+05 5.108E+04 8.52E+06 9.493E+09 NY-14-1 28.168 85.933 4265 606 6.1 2.7 0.978 0.0001 4.758E+05 2.617E+04 5.15E+06 6.592E+06 NY-14-2 28.168 85.933 4265 606 6.1 2.7 0.978 0.0001 6.200E+05 2.480E+04 n.a. n.a. NY-15 28.133 85.983 3890 635 3 2.7 1 0.0001 5.553E+06 2.332E+05 2.385E+07 1.919E+06 NY-16 28.133 85.983 3893 635 2.5 2.7 1 0.0001 9.630E+06 6.644E+05 3.239E+07 3.813E+06 NY-17 28.150 85.983 3891 635 1.5 2.7 1 0.0001 6.277E+06 1.883E+05 3.692E+07 3.359E+06 NAI-1 28.317 86.05 4289 604 3 2.7 0.974 0.0001 9.288E+05 5.579E+04 n.a. n.a. NAI-2 28.317 86.05 4308 603 3.9 2.7 0.974 0.0001 1.061E+06 3.394E+04 n.a. n.a. NAI-3 28.317 86.05 4343 600 5.9 2.7 0.974 0.0001 8.618E+05 4.678E+04 n.a. n.a. NAI-4 28.317 86.05 4310 603 4.5 2.7 0.974 0.0001 4.474E+06 2.909E+05 n.a. n.a. NAI-5 28.317 86.05 4280 605 1 2.7 0.974 0.0001 1.490E+06 6.342E+04 n.a. n.a.

To determine local airpressure we used an airpressure–altitude relation for the field region based on ECMWF re-analysis ERA40 data (see Fig. 4). ARTICLE IN PRESS 304 J.M. Schaefer et al. / Quaternary Science Reviews 27 (2008) 295–311

Table 1b Neon isotope data of all surface samples from the Nyalam boulders

Sample Temperature (1C), time (min) 20Ne (109 at/g) 21Ne/20Ne (103) 22Ne/20Ne

Ny-1 400, 45 3.0170.08 5.27070.061 0.102770.001 600, 30 2.6970.07 4.31070.066 0.102870.0011 1750, 15 3.6870.10 3.24470.032 0.101970.0012 Ny-2 600, 45 1.8670.08 9.3770.16 0.108470.0012 800, 20 1.3270.04 3.1970.10 0.103370.0011 1750, 15 1.1970.04 3.6270.11 0.101070.0015 Ny-3 400, 45 2.2670.06 10.7270.20 0.109670.0011 600, 30 2.3570.06 6.1270.25 0.102570.0005 1750, 15 2.4170.06 3.09270.055 0.099670.0007 Ny-4 400, 45 1.9170.05 8.09070.088 0.109670.0005 600, 30 2.15 70.06 4.28970.049 0.101170.0014 800, 30 2.3270.06 3.18770.059 0.098370.0008 1750, 15 3.8470.10 3.01970.048 0.101870.0005 Ny-5 400, 45 2.3370.06 14.4770.27 0.111970.0017 600, 30 1.2870.04 7.0770.21 0.104570.0014 1750, 15 1.8270.06 3.4170.14 0.101670.0021 Ny-6 400, 45 2.7870.12 17.0170.46 0.117770.0018 600, 30 3.0770.14 6.5370.19 0.106470.0019 800, 30 3.4870.12 3.4870.20 0.100570.0020 1750, 15 2.8270.14 3.6170.14 0.102770.0018 Ny-7 600, 45 14.570.36 4.14170.050 0.103370.0005 800, 20 11.070.36 3.36070.039 0.101970.0007 1750, 15 4.1770.13 3.67370.077 0.104070.0011

Ny-8 400, 45 1.5670.04 19.90070.088 0.121070.0008 600, 30 1.5070.05 6.15770.084 0.102870.0008 800, 30 1.8170.05 2.97670.051 0.101670.0009 Ny-15 400, 45 2.4770.07 11.0470.16 0.120570.0080 600, 30 2.4370.06 4.5570.15 0.104270.0005 1750, 15 2.7370.07 3.90370.036 0.102070.0004 Ny-16 400, 45 2.0370.06 15.7470.48 0.115170.0014 600, 30 2.4770.09 5.5870.12 0.105370.0017 800, 30 15.370.39 3.06470.043 0.102770.0007 1750, 15 3.8470.11 9.1170.14 0.100070.0015 Ny-17 600, 45 27.070.73 4.32570.081 0.102970.0068 800, 15 2.7970.07 3.2670.19 0.102070.0200 1750, 15 1.4270.04 2.9370.31 0.102070.0280

Stated uncertainties of 20Ne concentrations (1s) include statistical and sample weighing errors as well as variations of mass spectrometer sensitivity (typically 2–3%). Uncertainties of calibrations gas amounts are not included but should be o3%. Errors of isotopic ratios include statistical uncertainties of the ion counting and mass discrimination uncertainties. Temperature steps used for calculating the cosmogenic 21Ne excess are given in bold.

agreement between the four different scaling procedures We will discuss the new chronologies along their relative including distinct corrections for geomagnetic field stratigraphic position from young to old. fluctuations in time is good (differences up to 10% for the older ages, and only 3% for the young ages). In the 4.1. 10Be and 21Ne ages of glacial events discussion we rely on the results based on the Desilets et al. formalism (Desilets and Zreda, 2003) as implemented 4.1.1. Puluo moraines in the CRONUS-Earth webbased age calculator (last In agreement with their stratigraphic sequence, the columns in Table 2), which are very close to the ‘‘mean’’ 10Be ages of the outer Puluo 1 moraine are older than of the five scaling procedures applied. those from the inner Puluo 2 moraine (Fig. 7b). Ny-9 and All exposure ages are based on assumed erosion rate of Ny-10, the two samples from Puluo 1, yield 10Be ages 1mmky1 increasing the oldest ages by about 5%, the of 16.271.3 ky and 24.371.6 ky and suggest a deposition younger ages by 1% or less. of this moraine during MIS-2. The three samples from ARTICLE IN PRESS J.M. Schaefer et al. / Quaternary Science Reviews 27 (2008) 295–311 305

Table 1c Neon isotope data for the shielded samples from Nyalam boulders

Sample 20Ne ( 109 atoms g1) 21Ne/20Ne ( 103) 22Ne/20Ne 21Ne-excess ( 106 atoms g1)

Ny-3 (400, 450) 7.2170.17 3.2770.12 0.101070.0006 2.2171.05 Ny-3 (600, 300) 4.6570.14 3.3870.10 0.101270.0015 1.9770.71 Ny-3 (1750, 150) 27.5970.54 3.0470.04 0.101670.0006 2.1671.54 Ny-4 (400, 450) 3.0770.07 4.0070.03 0.101470.0006 3.2070.23 Ny-4 (600, 300) 2.6970.07 3.9470.02 0.101670.0005 2.6570.24 Ny-4 (800, 300) 2.3170.06 3.8370.03 0.102070.0005 2.0270.17 Ny-4 (1750, 150) 3.9870.08 3.3770.03 0.103170.0004 1.6470.20 Ny-6 (600, 300) 2.8070.13 5.2670.08 0.104070.0012 6.4470.33 Ny-6 (800, 300) 1.8070.08 3.0270.13 0.098470.0010 0.1170.28 Ny-6 (1750, 150) 2.9270.16 3.0970.09 0.100370.0007 0.3970.39 Ny-8 (400, 450) 2.3270.12 5.0870.05 0.104570.0014 4.9170.71 Ny-8 (600, 300) 2.8470.10 3.6670.16 0.103270.0019 1.9970.64 Ny-8 (800, 300) 2.7570.20 3.1970.11 0.101970.0021 0.6571.07 Ny-8 (1750, 150) 4.5570.13 3.0370.13 0.101870.0013 0.3370.76 Ny-15 (400, 450) 6.7970.14 4.6770.10 0.103370.0010 11.6170.82 Ny-15 (600, 450) 4.2870.11 4.3070.12 0.102170.0012 5.7370.67 Ny-15 (1750, 150) 5.4570.11 3.6570.12 0.100970.0015 3.7970.76 Ny-16 (400, 450) 4.9670.11 4.3070.11 0.100470.0008 6.6370.67 Ny-16 (600, 300) 55.9371.08 2.9070.02 0.097270.0005 0 Ny-16 (800, 300) 33.1370.58 2.9770.03 0.099070.0005 0.5271.28 Ny-16 (1750, 150) 23.8870.49 2.9970.03 0.098270.0007 0.8371.16

Temperature steps used to calculate cosmogenic 21Ne are highlighted in bold.

0.140

400°C/45' 600°C/30' 0.130 800°C/30'

Ny15 Ny8 Ny6

Ne 0.120 Ny16 20 Ny5

Ne/ Ny3

22 0.110 Ny2 0.100 y=(1.1200.021)x+0.098 AIR 0.090 0.0040 0.0080 0.0120 0.0160 0.0200 0.112

Ny4

0.108 Ny6 Ny11 Ny16

Ne Ny15 Ny5 20 0.104 Ny7 Ny11 Ny1

Ne/ Ny17

22 Ny4 Ny1 Ny8 Ny3 AIR 0.100

0.096 Fig. 6. 10Be and 21Ne ages for all surface samples. Neon (filled symbols) 0.0040 0.0060 0.0080 and beryllium (open symbols) exposure ages of the Nyalam erratics with 21Ne/20Ne 2s uncertainties (Table 2). The cosmogenic ages for Ny-4 and Ny-15 have been ‘‘corrected for pre-exposure’’ (Section 3.1). Boxes represent the three Fig. 5. Neon-3-isotope plots. Neon three-isotope plot of all Nyalam top different ridges of the sampled moraine set according to their stratigraphic samples. Most of the data points from 400 to 600 1C steps plot on the position old (outer) to young (inner). Ages do not show a trend to older atmospheric/cosmogenic mixing line. This makes non-cosmogenic con- values from the inner (right box) to the outer (left box) moraines, tribution unlikely and allows the calculation of exposure ages. The lower disagreeing with the stratigraphy. There is an apparent age cluster around panel shows the data points within the rectangle in the upper panel. Given 40–50 ky, while the rest of the ages are distributed uniformly between 100 uncertainties are 2s. and 190 ky. ARTICLE IN PRESS 306 J.M. Schaefer et al. / Quaternary Science Reviews 27 (2008) 295–311

4.1.2. Naisa moraines The three boulders NAI-1, NAI-2, and NAI-3 from the innermost and therefore youngest Naisa moraine display 10Be-ages of 15.471.2 ky, 17.370.7 ky, and 14.471.0 ky, with a mean of 15.771.5 ky. Those dates agree within uncertainties with the age of Ny-9 of 16.271.3 ky. NAI-5, from the outermost right lateral moraine yields a 10Be exposure age of 23.271.3 ky, indistinguishable from the age of Ny-10 of 24.371.6 ky. NAI-4 is an obvious outlier and is not further discussed. Similar to the Puluo moraines, the 10Be chronology of the two Naisa moraines suggests glacial events in the early and the late part of MIS-2 (Fig. 7b).

4.1.3. Nyalam moraines Cosmogenic 10Be and 21Ne isotope data and calculated exposure ages for the samples from the Nyalam moraines (Fig. 2b) are given in Tables 1 and 2,andFigs. 6 and 7a. The comparison of 10Be and 21Ne ages in Fig. 6 can be summarized as follows:

For 8 of the 11 samples plotted in Fig. 6, the beryllium and neon ages are in very good agreement. For Ny-16 and Ny-8, the 10Be ages are slightly higher than the 21Ne ages, but the ages still overlap within two sigma uncertainties. For sample Ny-6, however, the 10Be age is considerably higher, the reason for this discrepancy being unclear. Incomplete degassing of the cosmogenic 21Ne fraction cannot explain the offset of ages, because even if we base the 21Ne age on the total 21Ne excess above the atmospheric value using all four temperature degassing steps (Table 1b), the discrepancy would not decrease significantly. Alternatively, incomplete decon- tamination of the sample from meteoric 10Be would explain this effect, which appears unlikely given the well established procedures to process rock samples for cosmogenic 10Be. The cosmogenic chronology is at odds with the Fig. 7. Probability plots of the 10Be ages. (a) Summary of all samples included in this study. Colored lines are Gaussian probability distribution stratigraphic sequence, because the ages from the for individual samples, the thick black line is the sum of all individual stratigraphically younger inner moraine are higher than samples except to sample Ny-6 which is not plotted. The dominating age- those from the outer moraine (Fig. 6). cluster is visible during MIS-2 (see also b), another cluster during MIS-3 A cluster of ages exists in the range from 40 to 50 ky (Ny-1, Ny-2, Ny-4, Ny-7, Ny-15; see text). (b) MIS-2 samples. The (early MIS 3), the remaining boulder ages range from probability plot shows peaks during the Late Glacial period (Ny-11, Ny- 12, and Ny-13), during late MIS-2 at about 17 ky ago (the weighted mean 100 to 300 ky. of Ny-9, NAI-1, NAI-2, and NAI-3) and a lower peak at 23.5 ky (Ny-10, NAI-5). The outlier NAI-4 (65 ky) is not plotted. Because the exposure ages violate the stratigraphic sequence, the presented cosmogenic chronology from the Puluo 2, Ny-12, Ny-13, and Ny-14, yield very consistent boulders on top of the Nyalam moraines cannot be 10Be ages of 11.470.7 ky, 12.271.1 ky and 11.070.6 ky, explained by straightforward deposition of moraines by with a mean of 11.570.6 ky, suggesting a deposition of this successive glacial advances. Cosmogenic moraine chron- moraine during the Late Glacial period. ologies yielding a cluster of ages during early MIS-3 but The two 21Ne ages from the Puluo samples Ny-9 and Ny- also including much older ages, have been reported from 12 of 19.6711.7 ky and 16.1715.0 ky, respectively, are— other regions of the Himalayas (e.g. Phillips et al., 2000; within the large uncertainties, that are the consequence of Finkel et al., 2003; Owen et al., 2003). Typically, the older minute cosmogenic 21Ne excesses above the atmospheric ages are excluded as ‘‘pre-exposed outliers’’. We test the signal—consistent to the 10Be ages and argue against pre-exposure for our study site using samples from significant periods of pre-exposure. partially shielded samples. ARTICLE IN PRESS J.M. Schaefer et al. / Quaternary Science Reviews 27 (2008) 295–311 307

Table 2 Exposure ages based on five different scaling protocols

10Be-age (ky) 1s (ky) 10Be-age (ky) 1s (ky) 10Be-age (ky) 1s (ky) 10Be-age (ky) 1s (ky) 10Be-age (ky) 1s (ky) 21Ne-age (ky) 1s (ky)

Lal/Stone constant Lal/Stone variable Dunai variable Lifton variable Desilets et al. variable Desilets et al. variable

Ny-1 59.4 2.5 49.3 2.0 45.6 2.5 43.8 2.0 46.3 2.5 41.1 4.3 Ny-2 54.1 2.9 44.9 2.4 42.2 2.9 40.7 2.4 42.7 2.9 46.8 4.1 Ny-3B 126.0 4.5 103.3 3.5 96.2 4.4 92.2 3.5 97.8 4.5 97.8 9.5 Ny-4 74.4 4.1 63.1 3.4 58.9 4.1 55.8 3.3 59.9 4.2 52.2 4.8 Ny-5 200.9 8.8 159.7 6.5 141.9 7.6 135.5 6.1 147.3 8.0 118.3 9.0 Ny-6 476.9 30.8 351.9 19.1 305.3 20.8 289.1 16.4 317.2 22.0 162.9 23.9 Ny-7 59.8 4.7 49.6 3.8 45.6 4.3 43.8 3.6 46.4 4.4 64.9 7.1 Ny-8 190.8 10.1 151.5 7.6 134.4 8.7 129.2 7.1 139.1 9.0 113.2 5.0 Ny-9 17.2 1.1 16.7 1.0 16.7 1.3 15.6 1.1 16.2 1.3 19.6 11.7 Ny-10 27.5 1.4 25.4 1.3 24.7 1.6 23.3 1.3 24.3 1.6 n.a. Ny-12-1 11.7 0.5 11.5 0.5 12.0 0.7 11.1 0.5 11.4 0.7 16.1 15.0 Ny-12-2 11.5 0.5 11.3 0.5 11.8 0.6 11.0 0.5 11.3 0.6 n.a. Ny-13 12.6 0.9 12.4 0.9 12.8 1.1 11.9 0.9 12.2 1.1 35.5 39.5 Ny-14-1 8.6 0.5 8.4 0.5 9.1 0.6 8.4 0.5 8.5 0.6 22.0 28.2 Ny-14-2 11.2 0.5 11.0 0.4 11.5 0.6 10.7 0.5 11.0 0.6 n.a. Ny-15 130.1 6.3 106.2 4.9 98.6 6.0 94.9 4.9 100.4 6.1 92.2 7.4 Ny-16 253.5 23.3 196.5 16.6 179.5 18.6 170.9 15.4 184.4 19.3 117.3 13.8 Ny-17 147.7 5.2 118.3 4.0 108.7 4.8 104.7 3.9 111.2 5.0 137.4 12.5 NAI-1 16.3 1.0 15.8 1.0 16.0 1.2 14.9 1.0 15.4 1.2 n.a. NAI-2 18.6 0.6 17.9 0.6 17.8 0.8 16.7 0.6 17.3 0.7 n.a. NAI-3 15.0 0.8 14.7 0.8 14.9 1.0 13.9 0.8 14.4 1.0 n.a. NAI-4 84.3 6.0 70.8 4.9 64.0 5.5 61.1 4.6 65.1 5.7 n.a. NAI-5 26.0 1.1 24.2 1.0 23.6 1.3 22.3 1.1 23.2 1.3 n.a.

10Be surface exposure ages are based on the five different scaling procedures included in the CRONUS-Earth webbased calculator (http:// hess.ess.washington.edu/math/al_be_v2/al_be_multiple.php), Version 2.0 (Balco et al., in press). ‘‘constant’’ ¼ assumption of no geomagnetic field change with time. ‘‘variable’’ ¼ geomagnetic field change with time included. Details available under http://hess.ess.washington.edu/math/docs/al_be_v2/al_be_docs.html. Note that the significant difference between the ‘‘constant’’ and ‘‘variable’’ scaling, and the relatively good agreement of the different ‘‘variable’’ scaling procedures.

4.2. Control on pre-exposure through shielded samples Table 3 Comparison of calculated neutron fluxes J and measured nuclide Table 3 compares the calculated fluxes of the shielded concentrations C in shielded and exposed samples 21 6 1 a b and the exposed sample (Jbottom/Jtop) to the ratio of the Sample Necos ( 10 atoms g ) Jbottom/Jtop calc. Cbottom/Ctop meas. measured concentrations (Cbottom/Ctop) for the six Nyalam boulders for which partially shielded samples were Ny-3 4.1771.76 0.5470.16 0.1770.18 Ny-4 5.8570.47 0.3170.09 0.4670.09 analyzed. In four out of six samples (Ny-3, Ny-6, Ny-8, Ny-6 6.4470.33 0.3670.12 0.1370.02 and Ny-16), the measured ratios are lower than the Ny-8 6.9071.35 0.3870.11 0.2270.09 calculated values, arguing against pre-exposure. Whereas Ny-15 17.3571.49 0.5270.26 0.8970.20 7 7 7 for two of these boulders (Ny-8 and Ny-16), the agreement Ny-16 3.26 2.89 0.18 0.05 0.10 0.18 with the calculated values is satisfying, for Ny-6, and aCalculated with the mode described in the text. especially Ny-3, the shielded samples contain substantially bFor the bottom samples, the data of the same temperature steps as for less cosmogenic 21Ne than predicted. The most likely age calculation were used, not regarding their position in the three-isotope explanation for this effect is that the shielding of the plot. bottom samples decreased during deposition, either due to exhumation of the boulder or spalling off of a significant samples plot on the two component mixing line of slab overlying the shielded sample. This is in agreement atmospheric and cosmogenic neon in Fig. 5. This argues with the geomorphic field evidence (Section 2). against the presence of nucleogenic 21Ne and, in turn, In the two boulders Ny-4 and Ny-15, the bottom points towards a pre-exposure signal. The respective pre- samples contain more excess 21Ne than expected. This exposure period to produce the observed excess corre- suggests either the presence of nucleogenic neon or sponds to 15 and 50 ky, respectively. We use these numbers cosmogenic neon from a pre-exposure period. The 21Ne to correct the surface exposure ages of sample Ny-4 and ages of the two surface samples from these boulders agree Ny-15 to 45 and 50 ky, respectively, and assume conserva- well with the corresponding 10Be samples (Table 2) and the tively that this correction has an uncertainty of 50% ARTICLE IN PRESS 308 J.M. Schaefer et al. / Quaternary Science Reviews 27 (2008) 295–311 resulting in the large overall uncertainty for these samples visible in Fig. 6. This systematic test whether pre-exposure periods of boulders may be evaluated by means of partly shielded control samples yielded generally positive results. Most importantly, we conclude that no substantial non-in situ components are present in four of the six boulders. The respective exposure ages stated for Ny-3, Ny-6, Ny-8, and Ny-16 are thus stated true minimum ages. In the two others, we could estimate the pre-exposure signal to be rather small and corrected for the pre-exposure period. Future similar tests could be improved by more accurate determinations of boulder geometries.

Fig. 8. Landscape denudation event during MIS-3. Landscape denuda- 4.3. Snowline depressions for the Late Glacial and MIS-2 tion model proposed for the degrading moraines in monsoonal Tibet. glaciations Boulders successively exhume as the removal of till by surface runoff progresses. The ages of the blocks at the moraines surface at any time We calculated snowline depressions corresponding to the therefore range from zero to the age of moraine deposition. During a glacial advances Puluo 2 and Naisa. Because the terminal landscape denudation event, i.e. high degradation rates during relatively short period of time, more boulders than average become exhumed at the moraines are not preserved for Puluo 1 or Nyalam surface, producing a cluster of cosmogenic, exhumation ages, dating the glaciations, no snowline depression is calculated. Our denudation event. Here, we propose that the cluster of ages around THAR-method calculations (Section 2) yield snowline 40–50 ky from a much older moraine, is most easily explained by a depressions 250750 m for the Puluo moraines and landscape denudation event during warm and precipitation rich MIS-3 4507100 m for the Naisa glaciation. Applying an adiabatic (see text). lapse rate for this area of approximately 0.62 1C/100 m (Fig. 4), the recorded snowline depressions correspond to age pattern from the Nyalam moraine boulders may reflect temperature changes of 1.670.3 1C for the Late Glacial such continuous moraine degradation, as described by event and 2.870.6 1C for the MIS-2 event, respectively. Hallet and Putkonen (1994). By this scenario, the oldest ages on the moraines reflect the best minimum age estimate 5. Discussion for the moraine deposition indicating that the Nyalam moraines were deposited during the penultimate glacial 5.1. Landscape denudation in monsoonal Tibet during cycle or earlier. The cluster of ages between 40 and 50 ky MIS-3 reflects a period during which many boulders got exhumed within a relatively short period of time. We propose this to As shown above, the wide age scatter and the cluster of be a consequence of accelerated landscape denudation ages during early MIS-3 are not the result of exposure during precipitation-rich MIS-3. At times before and after, periods of the rocks prior to deposition on the moraine but boulders became exhumed at lower rates. rather suggest geological processes lowering the measured Similar clusters of surface exposure ages in early MIS-3 surface exposure ages relative to the real deposition age of were described in other regions of Tibet. Mostly, they were the respective moraine. Extending the basic idea given by interpreted to reflect glaciations, and it was postulated that (Zreda and Phillips, 1994), we propose the following the Last Glacial Maximum in Tibet had occurred as a scenario to explain the observed pattern: the exposure regional event, prior to the rest of the Northern hemisphere ages from the Nyalam moraines do not reflect the (Benn and Owen, 1998; Phillips et al., 2000; Richards et al., deposition of the respective moraine. Instead, we suggest 2000; Finkel et al., 2003; Owen et al., 2003). In view of our a model of moraine degradation and denudation in parallel data, however, it seems possible that these earlier data to boulder spalling (Fig. 8). Precipitation, as the main reflect the same denudation event proposed here. agent of moraine degradation (Allen, 1997) particularly in combination with large temperature fluctuations, can erode 5.2. Glaciations during MIS-2 and the Late Glacial period the surface of a moraine by meters within as little as 10 ky (Hallet and Putkonen, 1994). During this process, loose Our results from the Puluo and Naisa moraines indicate material is removed from the moraine, boulders succes- that the timing of glacial events during the last deglaciation sively emerge from the till and may become exposed to in the monsoonal Himalayas is generally consistent with cosmic radiation and weathering well after deposition. the ‘‘classic’’ pattern characterized by a modest Last Accordingly, the exposure ages of boulders found on the Glacial Maximum advance during MIS-2 and a less surface of a moraine range from zero up to the moraine extensive and younger Late Glacial advance (Fig. 7b). age. We conclude that in agreement with the geomorphic The cosmogenic chronologies reported here are in agree- appearance of the Nyalam moraine ridges, the exposure ment with earlier studies from the eastern margin of Tibet ARTICLE IN PRESS J.M. Schaefer et al. / Quaternary Science Reviews 27 (2008) 295–311 309

Puluo/Naisa (n=6)

Karola Pass (n=11) Hunza Valley (n=7)

Mt. Everest (n=4)

Litang (n=6)

NH mean

SH mean

-360 Antarctica 280 Termination I -380 EPICA Dome C

-400 240 (ppmv) 2

D ( ) -420 δ CO

-440 200

-460

10 15 20 25 30 Age (ky)

Fig. 9. MIS-2 glaciations in Tibet and their inter-hemispheric context. The upper panel shows a compilation of 10Be data from LGM moraine ages in Tibet (Puluo/Naisa: this study including samples Ny-9, Ny-10, NAI-1, NAI-2, NAI-3, and NAI-5); Karola Pass (Owen et al., 2005); Hunza Valley (Owen et al., 2002b); Mt. Everest (Finkel et al., 2003); Litang (Scha¨fer et al., 2002), compared to the timing of the LGM termination moraines in southern- and northern mid-latitude mountain glacier systems (Schaefer et al., 2006). The lower panel shows the deuterium data (as a proxy for mean annual temperature; Jouzel et al., 2001) and the atmospheric CO2 concentration from the Dome C ice core (Monnin et al., 2001) in Antarctica.

(Scha¨fer et al., 2002; Tschudi et al., 2003), influenced by the groundwater-noble gas studies from monsoonal Asia East Asian monsoon system (Fig. 9). Both findings imply (Mekong Delta, Southern Vietnam, 101N, 1061E(Stute that the Last Glacial Maximum as well as the Late Glacial et al., 1997). This latter value is supported by similar advance in monsoonal Tibet were plateau-wide events. Our studies from Brazil (Stute et al., 1995) and Niger (Beyerle chronologies yield evidence that the onset of deglaciation et al., 2003). On the other hand, recent tropical sea-surface after the Last Glacial Maximum was consistent to the temperature reconstructions using Mg/Ca-ratios in for- termination that has been reported from mid-latitude aminifer shells indicate a drop of less than 3 1C during the glaciers on both hemispheres (Fig. 9), near-synchronous Last Glacial Maximum compared to today (Lea et al., to the temperature warming and increase in atmospheric 2000). Within given uncertainties our data is consistent CO2 concentrations recorded in Antarctic ice cores with both, the atmospheric and the sea surface temperature (Schaefer et al., 2006). The scenario that glaciers in highly gradients between full glacial conditions and today’s monsoonal Tibet respond near-synchronously to the onset temperature, and supports the view that the earlier of warming triggering the inter-hemispheric termination of reported CLIMAP Last Glacial Maximum temperature mid-latitude glaciers supports the view that late Pleistocene decrease of about 1 1C is too low. glaciations were mainly driven by summer temperature changes (Oerlemans, 2005; Anderson and Mackintosh, 6. Conclusions 2006) rather than by increases in precipitation. We also present estimates of snowline depressions for Cosmogenic nuclides 10Be and 21Ne show that Nyalam both of the Late Pleistocene advances. The temperature moraines in monsoonal south Tibet have been deposited decrease underlying the MIS-2 glacial advance (Naisa during the penultimate glaciation or even earlier. Glaciers glaciation) of some 3 1C relative to today is slightly less did not reach a similarly large extension afterwards. This than the cooling of the tropical atmosphere during the Last finding is consistent with results from the dry Central Glacial Maximum of approximately 5 1C as reported from Plateau (Scha¨fer et al., 2002; Owen et al., 2005) as well as ARTICLE IN PRESS 310 J.M. Schaefer et al. / Quaternary Science Reviews 27 (2008) 295–311 from the Ladakh Range, northern India (Owen et al., References 2006). The large scatter of boulder ages from the Nyalam moraines, a typical feature for TCN studies in Tibet, is Abramowski, U., Bergau, A., Seebach, D., Zech, R., Glaser, B., Sosin, P., interpreted to be caused mainly by successive exhumation Kubik, P.W., Zech, W., 2006. Pleistocene glaciations of Central Asia: results from 10Be surface exposure ages of erratic boulders from the of blocks from the moraines. By this scenario, the Pamir (Tajikistan), and the Alay–Turkestan range (Kyrgyzstan). generally humid climate overprinted by high millennium- Quaternary Science Reviews 25, 1080–1096. scale variability during early MIS-3 caused a ‘‘landscape Allen, P.A., 1997. Earth Surface Processes. Blackwell Science Ltd., denudation event’’, accelerating exhumation and Oxford, p. 396. leading to an accumulation of boulder ages between 40 Alley, R.B., 2003. When Earth’s freezer door is left ajar. EOS 84, 315. and 50 ky. Anderson, B., Mackintosh, A., 2006. Temperature change is the major driver of late-glacial and Holocene glacier fluctuations in The data presented from the Late Glacial and MIS-2 New Zealand. Geology 34, 121–124. moraines indicate that the timing of the major late Balco, G., Schaefer, J.M., 2006. Cosmogenic-nuclide and varve chron- Pleistocene glaciations in monsoonal Tibet were generally ologies for the deglaciation of southern New England. Quaternary consistent with mid-latitude glaciations and occurred Geochronology 1, 15–28. during periods of relatively cool and dry climate (Abra- Balco, G., Stone, J.O., Lifton, N.A., Dunai, T.J., in press. A complete and easily accesible means for calculating surface exposure ages or erosion mowski et al., 2006). This contradicts the view that rates from 10Be and 26Al measurements. Quaternary Geochronology. increased moisture supply by intensive monsoon has been Baur, H., 1999. A noble gas mass spectrometer compressor source with the main driver of glaciations in Tibet. We propose that two orders of magnitude improvement in sensitivity. EOS Transac- glaciers in Tibet and elsewhere expand significantly when tions AGU 80 (Supplement). the melting during the ablation season is reduced by cooler Benn, D.I., Owen, L.A., 1998. The role of the Indian summer monsoon summer temperatures (Oerlemans, 1994, 2001; Alley, 2003; and the mid-latitude westerlies in Himalayan glaciations: review and speculative discussion. Journal of the Geological Society, London 155, Denton et al., 2005). 353–363. The snowline depressions of 4507100 m and 250750 m Beyerle, U., Aeschbach-Hertig, W., Imboden, D.M., Baur, H., Graf, T., for the two dominant latest Pleistocene glaciations in Kipfer, R., 2000. A mass spectrometric system for the analysis of noble Nyalam are smaller than those from mid-latitudes (typi- gas and from water samples. Environmental Science and cally about 1200 m snowline depression compared to Technology 34, 2042–2050. Beyerle, U., Ruedi, J., Leuenberger, M., Aeschbach-Hertig, W., Peters, F., the current glacier position related to the Last Glacial Kipfer, R., Dodo, A., 2003. Evidence for periods of wetter and cooler Maximum (e.g. Porter, 1975; Maisch, 2000) and about climate in the Sahel between 6 and 40 kyr BP derived from ground- 400–500 m related to the Late Glacial (e.g. Dahl and Nesje, water. Geophysical Research Letters 30, 1173. 1992). The cooling derived here from snowline depressions Dahl, S.O., Nesje, A., 1992. Paleoclimatic implications based on in southern Tibet for the MIS-2 glaciation of about 3 1Cis equilibrium-line altitude depressions of reconstructed Younger Dryas consistent with sea surface temperature reconstructions and Holocene cirque glaciers in inner Nordfjord, western Norway. Paleogeography, Paleoclimatology, Paleoecology 94, 87–97. from the equatorial Pacific (Lea et al., 2000), but is Denton, G.H., Alley, R.B., Comer, G.C., Broecker, W.S., 2005. The role considerably larger than the 1 1C Last Glacial Maximum of seasonality in abrupt climate change. Quaternary Science Reviews cooling assumed by CLIMAP for the tropical Last Glacial 24, 1159–1182. Maximum. Desilets, D., Zreda, M., 2003. Spatial and temporal distribution of If indeed glaciations world-wide (excluding Antarctica) secondary cosmic-ray nucleon intensities and applications to in situ were driven predominantly by changes in summer tem- cosmogenic dating. Earth and Planetary Science Letters 206, 21–42. Dunne, J., Elmore, D., Muzikar, P., 1999. Scaling factors for the rates of perature, this would enable us to perform detailed summer- production of cosmogenic nuclides for geometric shielding and paleothermometry on a near-global scale by mapping and attenuation at depth on sloped surfaces. Geomorphology 27, 3–11. dating past glacial advances. Finkel, R.C., Owen, L.A., Barnard, P.L., Caffee, M.W., 2003. Beryllium- 10 dating of Mount Everest moraines indicates a strong monsoon Acknowledgments influence and glacial synchroneity throughout the Himalaya. Geology 31, 561–564. 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